1. Field of the Invention
The invention relates to a friction lining and a process for producing a friction lining, in particular for dutch disks, brakes or the like.
2. Description of Prior Art
Such clutch linings are used, in particular, for friction clutches in motor vehicles in which they are riveted onto a metallic support plate or clutch disk in order to allow, in combination with a counter-element in the clutch-engaged state, transmission of force between an engine and a gearbox or to disengage in the case of an open clutch.
The clutch linings here have to meet demanding requirements because they need to meet a plurality of requirements which cannot readily be achieved in the same way. In particular, the clutch lining should have a high burst rotational speed in order not to be destroyed by centrifugal forces, have a high coefficient of friction by means of which a large torque can be transmitted, also display low wear and be resistant to shape-changing distortion as a result of heat and/or forces. The clutch linings should nevertheless be very uncomplicated and thus be economically producible.
According to the prior art, a winding is first produced in order to produce a clutch lining. Such a winding can, for example, be wound from an impregnated yarn. Here, a yarn optimized in terms of coefficient of friction and wear is drawn through an impregnation solution and brought to a constant weight ratio of yarn to impregnation solution. The yarn is subsequently dried. For example, a carded yarn comprising aramid fibers, staple glass fibers, viscose fibers or PAN fibers, and also brass wire can be used. Another example is a mixed filament yarn comprising glass filament and metal wires. The various yarns can also be used in combination, The impregnation solution used is a dispersion comprising polymeric components, fillers and solvents. The polymeric components can comprise thermosetting components such as phenolic resin or melamine resin and also elastomeric components such as SBR or NBR rubber. Fillers are, for example, barium sulfate, kaolin or carbon black. The solvent used is usually water. The dried yarn is subsequently wound to form a winding.
Novolak resins are predominantly used as binders for producing known frictional linings according to the prior art. Apart from the novolak-based binders, phenolic and cresol resols are also used. In the case of compositions which are produced moist, solvent-containing and aqueous resols are predominant. An important parameter in the case of these liquid binders is that they have a high impregnation capability, uniform drying and setting rates and good compatibility with any rubber modifiers used. A disadvantage of the known binders based on synthetic resin is that they do not have a satisfactory heat stability.
According to the prior art, organically bound friction linings are used for dry-running clutches. The term “organically bound” is derived from chemical nomenclature and indicates that phenolic resins, melamine resins, polyester resins and/or various types of rubber are used as binders. As phenolic resin, use is usually made of a resol, viz. a self-cross-linking phenolic resin based on water as solvent. As an alternative, solvent-containing novolak resin systems are used. These binders are provided with a friction-active, usually powdery filler system and applied as a dispersion to a yarn comprising organic and/or inorganic fibers in an impregnation process with subsequent drying. This impregnated yarn treated with “friction cement” is wound into a defined shape and pressed in the subsequent process steps. Finally, final machining is carried out.
Resol systems in particular frequently react extremely unflexibly to raw material modifications, in particular modifications of the rubber system (latex compatibility). Continuous matching of the formulations to changing market conditions can thus be achieved only with difficulty.
A further disadvantage of the resol system is the high water or solvent content of the resol or novolak and also of the added latex. The water or the solvent has to be removed again by means of a system which consumes a great deal of energy or heat. During this heating, in particular in the case of resol, there is a risk that the self-crosslinking phenolic resin reacts in an undefined manner and thus alter the materials properties. The reaction products formed on cross-linking have to be disposed of because of their toxicity, usually by means of thermal after-combustion, in a costly manner.